The present invention concerns the production of lead powder for the dry filling of tube electrodes for storage batteries.
Tube electrodes for lead storage batteries are known and have been described, for example, in U.S. Pat. No. 2,747,007. They are constructed from a grid of lead or a lead alloy, which grid contains a so-called top frame, from which extends a number of conducting bars or rods, generally of the same lead composition as the grid. Each rod is surrounded by active material and a tubular sheath of inactive porous material. The active material of fully charged positive electrodes consists of lead dioxide. In the production of the electrodes, the tube sheaths are set on the grid which is then turned upsidedown on the top frame and thereafter the active material, which is provided in the form of so-called lead powder, is charged into the sheaths. The lead powder used in filling the sheaths is lead powder of varying degrees of oxidation and generally from about 70 up to 100% oxidation. The filling proceeds as follows: the electrode with the sheaths set about the rods is set up in a cassette which at the top is provided with a small-sized funnel. Thereupon, the cassette is vibrated and the lead powder which is to be conveyed to the electrode is filled down in the funnel and through the funnel into the space between the sheath and the rod. Finally, the electrode is closed with a bottom strip which covers all the tube apertures.
The dust binding of the lead powder which arises in connection with the production of tube electrodes according to the above process is very inconvenient and causes great problems. It is therefore very important that the dust binding be avoided or be reduced to a minimum. In order to reduce or eliminate dust binding, various filling methods which utilize wet mass mixtures have been tried. For example, a mass of the lead dust and water and/or other additions in small amounts may be prepared. This mass is then conveyed to an extruder or some similar feeding device and is pressed into the tube sheaths. Such a procedure is described, for example, in Swedish patent application No. 7408864. While this wet process reduces the dust binding, it produces other problems such as the formation of an irregular filling of the electrodes and increased amounts of lead in the waste water.
Polytetrafluoroethylene (PTFE) is also used for dust binding purposes not connected with the production of electrodes. In such a process, up to two percent in weight of PTFE are added to the dust-bonding powder. PTFE is present either as a dry powder or in suspension. The mixture is then heated to a temperature preferably just under 100.degree. C and is processed. Thereby the PTFE changes from a powder to fibers.
In connection with the production of lead accumulator electrodes in the dry powder process described above, various additions to the lead dust have been attempted for different purposes. Silicon dioxide and sodium sulphate are examples of such additions. These additions, however, have had negative effects on the life span of the electrodes.